1. Field of the Invention
The present invention relates to a pulse wave processing apparatus and a pulse wave processing method for processing pulse wave signals as data.
2. Description of the Related Art
An example of biomarkers that prominently reflect the state of a subject is heartbeats (e.g., autonomic nerve activities based on analysis of heartbeat rates and frequencies of heartbeat intervals). Electrocardiography is commonly used to measure heartbeats directly. To perform electrocardiography, it is necessary to paste electrodes on the subject in, for example, a number of places on the chest. The subject is greatly burdened if he/she needs to have the electrodes attached to his/her body all the time while he/she leads his/her daily life. Accordingly, the pulse waves on fingers, wrists, and earlobes are used as biomarkers that can be obtained more easily and are equivalent to heartbeats. However, a problem arises where, when pulse waves are measured, the waveforms are more greatly influenced and disturbed by movements of the body (hereinafter “body movements”) than when electrocardiography is used. To cope with this problem, techniques for eliminating body movement influences from the waveforms of the pulse waves have been proposed (see, for example, Japanese Patent No. 2816944, JP-A 2001-61795 (KOKAI), JP-A 2002-17694 (KOKAI), JP-A 2005-95653 (KOKAI), and JP-A 2005-160640 (KOKAI)).
According to the techniques disclosed in these documents, in principle, body movement components are eliminated from the characteristics of the pulse wave signal or from the characteristics of the pulse wave signal and the body movement signal within a predetermined section, so that the average pulse rate for the section can be calculated. These techniques are suitable for the use during physical exercises; however, in the case where the body movement signal and the pulse wave signal are in substantially the same frequency bandwidth, a problem remains where it is difficult to eliminate the body movement components. Also, it is not possible to use these techniques for the purpose of extracting the interval for each pulse beat of the pulse wave and performing an autonomic nerve analysis based on an analysis of the frequency of the fluctuation component.
On the other hand, there is another method for judging, depending on how large body movements are, whether each of parts of the pulse wave that are obtained when the body movements were made should be used in the analysis or not. According to this method, it is possible to judge whether each of the parts of the pulse wave should be used or not in correspondence with each pulse beat; however, in the case where a body movement that exceeds a predetermined magnitude is made, there is a possibility that all of the parts of the pulse wave may be judged to be unusable, and even some parts of the data that properly express the measured pulse wave may be lost. To perform an autonomic nerve analysis with a high level of precision, it is desirable to use as large a part of the data that correctly expresses the measured pulse wave as possible.